1. Field of the Invention
The invention relates to carbon-matrix composites, and in particular embodiments, to carbon-matrix composites derived from thermoplastic polyacrylonitrile fibers.
2. Description of the Related Art
Excellent high temperature performance characteristics of carbon composites in structural, frictional, ablative, and thermal insulation applications have caused an ever expanding demand for such materials. Accordingly considerable effort has been extended in recent years towards developing new techniques for the large scale production of such materials for use in nuclear, aerospace, aircraft and industrial fields.
Carbon-matrix composites are materials that are composed of a fibrous reinforcement in a carbonaceous or graphitic matrix. A filler or coating may also be included to impart specialized properties. Carbon-carbon parts, wherein the fibers and the matrix are both carbon-based, have been put to a variety of uses, including aeronautical and space applications, because of their light-weight and high temperature properties. Carbon-carbon composites are lightweight materials, with densities ranging up to about 2.00 g/cm3, depending on the precursors used for their production. Carbon-matrix composites may have greater density, depending on the density of the particular fibrous reinforcement used. Carbon-matrix composites possess great thermal stability in non-oxidizing environments and may be coated with an oxidation-resistant coating for use in oxidizing environments. Carbon-carbon components are also desirable because of their resistance to high temperature and thermal shocks, coupled with high temperature strength.
The carbon fibers in a carbonxe2x80x94carbon composite are generally derived from three main precursors; namely, rayon, polyacrylonitrile (PAN), and pitch. The use of rayon precursor has been largely abandoned in recent years because of the resulting poor quality of the carbon fibers produced. Currently, fiber manufacturers generally use PAN- or pitch-based precursors. PAN is often preferred for high strength, whereas pitch derivatives are desirable for high modulus and high thermal conductivity.
The use of carbonxe2x80x94carbon composites in engine components in the industrial and automotive market has not been extensive, primarily for two reasons. The first is cost of carbon fiber itself. In the early 1990""s, carbon fiber cost about $40/lb, and now costs $8-9/lb, and the near term projections are for under $5/lb. This cost reduction and projected increased demand for fibers should drive the fiber cost down further, making the carbonxe2x80x94carbon composites likely to replace steel and aluminum in many applications.
The second reason why carbon-matrix composites, such as carbonxe2x80x94carbon composites, have not achieved great commercial success is the difficulty encountered in trying to optimize and reduce the cost of the fabrication process. In order to yield the desired composite properties, multi-step processing techniques may be utilized to convert the binder into carbon matrix or add carbon matrix via chemical vapor deposition. Traditional processing consists of mixing the fiber with resin and shaping preforms into the desired shape. These shapes or preforms are kept in a high temperature furnace and heat treated for several hours ranging from 800 to 2000xc2x0 C. After firing, the composites or performs are placed in a CVD furnace and densified. CVD refers to chemical vapor deposition. Due to the nature of CVD, it is extremely difficult to fabricate thick specimens with uniform density. As such, even for thin samples the CVD process can take from a few days to several weeks to finish. The time costs have made these processes highly labor intensive and not conducive to high volume production.
A variety of methods and materials for making carbonxe2x80x94carbon composites are described in numerous publications and patents including, for example, the following: Buckley, John D. and Edie, Dan D., ed., Carbonxe2x80x94Carbon Materials and Composites, Noyes Publications, Park Ridge, N.J. (1993); Delmonte, John, Technology of Carbon and Graphite Fiber Composites, Van Nostrand Reinhold Company, New York, N.Y. (1981); Schmidt et al, xe2x80x9cEvolution of Carbonxe2x80x94Carbon Composites (CCC)xe2x80x9d SAMPE Journal, Vol.32, No. 4, July/August 1996, pp 44-50; xe2x80x9cExpanding Applications Reinforce the Value of Compositesxe2x80x9d High Performance Composites 1998 Sourcebook; U.S. Pat. No. 3,914,395 to Finelli, et al; U.S. Pat. No. 4,178,413 to DeMunda; U.S. Pat. No. 5,061,414 to Engle; U.S. Pat. No. 4,554,024 to Zimmer, et al; and U.S. Pat. No. 5,686,027 to Olsen, et al.
The present invention includes carbon-matrix and carbonxe2x80x94carbon composites, and systems and methods for manufacturing such composites. For instance, in certain embodiments, it is an object of the present invention to provide a novel method of making a carbon-matrix composite product in which a precursor substrate is constructed from stabilized carbon precursor reinforcing fibers and thermoplastic fibers. The precursor substrate may be compressed under controlled conditions of, e.g., temperature and pressure to fuse the thermoplastic fibers to the reinforcing fibers, and the precursor substrate may then be carbonized.
Thus, in one aspect the invention provides a composite comprising thermoplastic fibers and stabilized carbon precursor reinforcing fibers, wherein the thermoplastic fibers and the reinforcing fibers are carbonized to yield the composite. In certain embodiments, the reinforcing fibers can comprise oxidized polyacrylonitrile fibers. In certain embodiments, the thermoplastic fibers are high char yield fibers, such as phenolic resin, pitch, epoxy resin, phthalonitrile resin, aromatic acetylene-derived polymer, or unoxidized polyacrylonitrile fibers. In certain embodiments, the thermoplastic fibers comprise unoxidized polyacrylonitrile and, for example, the reinforcing fibers comprise oxidized polyacrylonitrile. In certain embodiments, the fibers have substantially identical lengths, or lengths which differ by at least about 50%, at least about 300%, or at least about 1000%. The thermoplastic fibers and the reinforcing fibers may be provided as substantially individual fibers, or as yarns, e.g., separate yarns, or a yarn comprising thermoplastic and reinforcing fibers.
In a second aspect, the composite may be a woven fabric, which the reinforcing fibers are interwoven with the thermoplastic fibers. Alternatively, the composite may be a non-woven fabric in which the reinforcing fibers are blended with the thermoplastic fibers in a generally non-ordered manner.
In another aspect, the invention provides a composite prepared by combining thermoplastic fibers and stabilized carbon precursor reinforcing fibers, fusing the thermoplastic fibers to the stabilized reinforcing fibers and carbonizing the fibers to produce a composite. In certain embodiments, the method may include stabilizing and/or oxidizing the thermoplastic fibers, preferably prior to carbonizing the fibers. The thermoplastic fibers and the reinforcing fibers may be selected as described above. The reinforcing fibers and thermoplastic fibers may be combined by blending the fibers to form a non-woven fabric. Alternatively, the reinforcing fibers and the thermoplastic fibers can be interwoven to form a woven fabric.
In another embodiment, the invention provides a method for forming a composite comprising stabilizing a plurality of carbon precursor fibers, combining the stabilized carbon precursor fibers with the thermoplastic fibers, fusing the thermoplastic fibers to the carbon precursor fibers, and carbonizing the thermoplastic and the reinforcing fibers to produce a composite. The fibers may be combined to form a woven fabric, wherein the thermoplastic fibers are interwoven with the carbon precursor fibers, or may be combined to form a nonwoven fabric. The thermoplastic fibers and the carbon precursor fibers may be selected as described above. In certain embodiments, the method includes the optional step of stabilizing and/or oxidizing the thermoplastic fibers prior to carbonizing the fibers.
In further aspect, the invention provides a method for preparing a composite by forming a precursor substrate comprising polyacrylonitrile fibers and oxidized polyacrylonitrile reinforcing fibers, fusing the polyacrylonitrile fibers to the reinforcing fibers to form a polyacrylonitrile matrix, and carbonizing the polyacrylonitrile fibers and the reinforcing fibers to form a composite. In certain embodiments, the method includes the optional step of stabilizing and/or oxidizing the polyacrylonitrile fibers prior to carbonizing the fibers. The unoxidized polyacrylonitrile fibers and the oxidized polyacrylonitrile fibers may be provided as substantially individual fibers, or as yarns, e.g., separate yarns, or a yarn comprising thermoplastic and reinforcing fibers. The precursor substrate may be a woven fabric in which the polyacrylonitrile fibers are interwoven with the reinforcing fibers, or a non-woven fabric.